Hot melt adhesive compositions are solid at room temperature but, upon application of heat, melt to a liquid or fluid state in which molten form they are applied to a substrate. On cooling, the adhesive composition regains its solid form. The hard phase(s) formed upon cooling of the adhesive composition impart all of the cohesion (strength, toughness, creep and heat resistance) to the final bond. Hot melt adhesive compositions are thermoplastic and can be heated to a fluid state and cooled to a solid state repeatedly. Hot melt adhesive compositions do not include water or solvents.
Curable or reactive hot melt adhesive compositions are also solid at room temperature and, upon application of heat, melt to a liquid or fluid state in which molten form they are applied to a substrate. On cooling, the adhesive composition regains its solid form. The phase(s) formed upon cooling of the adhesive composition and prior to curing impart initial or green strength to the bond. The adhesive composition will cure by a chemical crosslinking reaction upon exposure to suitable conditions such as exposure to moisture. Before curing the adhesive composition remains thermoplastic and can be remelted and resolidified. Once cured, the adhesive composition is in an irreversible solid form and is no longer thermoplastic. The crosslinked adhesive composition provides strength, toughness, creep and heat resistance to the final bond. Hot melt curable adhesive compositions can provide higher strength and heat resistance compared to non-curable hot melt adhesive compositions.
The ability of a reactive hot melt adhesive composition to cool so that the solidified but non-crosslinked composition can quickly bond parts together is called green strength. An adhesive composition that quickly develops green strength is desirable in commercial operations as it allows bonded parts to be further processed quickly. Reactive hot melt adhesive compositions will cure under appropriate conditions such as exposure to moisture so that strength of the adhesive bond between parts will continue to rise. A high cured strength is desirable in commercial operations as it allows stressed parts to be bonded.
In some applications such as roller coating the adhesive composition is melted in the reservoir of roller coating equipment and applied as a thin film by a roller to a substrate. The molten adhesive composition in the roller coating equipment will react with moisture in the air and begin to crosslink. At some time the cross linking will progress to a point where the equipment must be shut down so the partially cross linked adhesive composition can be removed and the equipment cleaned. Failure to clean the partially crosslinked adhesive composition can lead to application difficulties and ultimately to the composition fully curing and solidifying in the equipment, requiring equipment shutdown and extensive disassembly. Thus, a long working life is desirable.
Some adhesive compositions will form a string between the just coated substrate and application equipment such as a roller coater when the coated substrate is removed from the equipment. These hot adhesive strings are undesirable as they accumulate on the equipment and require cleaning. Thus, minimizing stringing is desirable. These requirements are conflicting. An adhesive composition that crosslinks quickly to provide cured strength will have a short working life. An adhesive composition that crosslinks slowly will have a long working life but will develop strength more slowly, slowing subsequent commercial operations. It can be difficult to find one reactive hot melt adhesive composition that has a commercially desirable combination of green strength, cured strength, working life and stringing
The majority of reactive hot melts are moisture-curing urethane hot melt compositions. The reactive components of urethane hot melt compositions consist primarily of isocyanate terminated polyurethane prepolymers containing urethane groups and reactive isocyanate groups that react with surface or atmospheric moisture to chain extend and form a new polyurethane polymer. Polyurethane prepolymers are conventionally obtained by reacting diols with diisocyanates. Upon cooling the isocyanate groups in the polyurethane prepolymer react with moisture from the environment to form a crosslinked irreversible solid bond.
Moisture-curing urethane hot melt adhesive compositions have certain disadvantages. One disadvantage is the residual monomer content of polyisocyanates, more particularly the more volatile diisocyanates. Some moisture-curing urethane hot melt adhesive compositions can contain significant amounts of unreacted monomeric diisocyanates. At the hot melt application temperature (typically at 100° C. to 170° C.) monomeric diisocyanates have a considerable vapor pressure and may be partly expelled in gaseous form. The isocyanate vapors may be toxic, irritating and have a sensitizing effect, so that precautionary measures have to be taken in the application process.
Silane reactive hot melt adhesive compositions have been developed to avoid disadvantages of isocyanate reactive hot melt compositions. Silane reactive hot melt adhesive compositions are also solid at room temperature and, upon application of heat, melt to a liquid or fluid state in which molten form they are applied to a substrate. On cooling, the composition regains its solid form. Silane reactive hot melt adhesive compositions are based on silane modified polymers that comprise moisture reactive silane groups that form siloxane bonds when exposed to moisture such as in the atmosphere. Silane reactive hot melt adhesive compositions can be free of residual isocyanate monomers and isocyanate functionality. Silane reactive hot melt adhesive compositions offer good cured adhesion and since there is no isocyanate there are no concerns about emission of isocyanate monomer vapor. However, silane reactive hot melt adhesive compositions develop green strength slower than reactive polyurethane hot melt adhesive compositions.
There remains a need for a silane reactive hot melt adhesive composition that has a desirable combination of properties for commercial use including quick development of green strength, a long working life and high final (cured) strength.